1. Field of the Invention
The present invention relates to conveyor belts and, in particular, to a conveyor belt having an internal stretch limiting means disposed only in the center portion of the belt to allow a controlled amount of tension in the belt while permitting the belt edges to remain in tension when traversing horizontal curves.
2. Description of the Prior Art
As is known, conveyor belts have been developed for travelling through an orbit with a generally horizontally extending conveying run overlying a generally horizontally extending return run with the conveying and return runs being stretched or elongated to provide tension in the sides of the conveying run so that such sides remain in tension as the conveying run traverses a horizontal curve. A description of such a tensioned conveyor belt is set forth in U.S. Pat. No. 4,387,801 wherein one embodiment of a conveyor belt is disclosed and described having a selected limited elongation for establishing the overall length of an orbital conveyor belt. Such conveyor belt elongation is obtained by fastening an orbital drive chain to the resilient conveyor belt with the belt in its relaxed or slacked condition, i.e., without any longitudinal tension being applied to the belt runs, with a specific spatial relationship between adjacent links of the chain. When such a chain is longitudinally tensioned, as is the case when supported for travel through an orbital path, the slack between adjacent links of the chain is eliminated or taken up and the chain becomes taut. The slack eliminating tension of the chain also simultaneously elongates the conveyor belt to the same linear extent as the chain is elongated when moving from a slack to a taut condition. Thus, a preselected elongation is achieved in the conveyor belt as determined by the total slack between adjacent links of the chain.
A somewhat similarly functioning conveyor belt is disclosed in U.S. Pat. No. 4,474,289. In such patent, a control member is provided to allow for a uniform longitudinal tension in a conveyor belt. Specifically, the control member comprises an elongated endless multi-section strap consisting of bowed portions located between anchored or fixed portions which fixed portions are secured to the underside of an orbital elongatable conveyor belt. The bowed sections of the strap form a series of bowed portions throughout the length of the conveyor belt such that upon proper tensioning, stretching or elongation of the belt and strap assembly for orbital movement, the longitudinal movement of the strap causes each of the bowed portions to flatten and extend longitudinally between adjacent fixed portions with the maximum orbit length of the conveyor belt being determined by the length of the so-flattened strap.
An alternative form of conveyor belt is shown in U.S. Pat. No. 4,410,082. In certain embodiments of the conveyor belts disclosed in such patent, there is provided across the belt cross section a layer of square woven stretch fabric. Such layer extends from one edge of the belt to the other and has the property of stretching a predetermined distance when subject to tension during the initial stretching of the belt and then its resistance to further elongation increases rapidly to resist any appreciable stretching of the belt in operation.
In U.S. Pat. No. 4,770,290, there is disclosed an apparatus and method that produces a conveyor belt made of polyurethane wherein longitudinal reinforcement strands are disposed across the entire width of the belt body proper to allow for a limited stretching of the belt. The strands are given a wavy or serpentine preform which allows the desired stretch of the conveyor belt. As an alternative, a single ply fabric with sufficient wavy resiliency in its weave may be cast within the body of the polyurethane belt to provide for limited stretching of the conveyor belt. However, in such embodiment, the fabric is disclosed as having a square weave and traverses the entirety of the belt cross section. A suggested modification of the wavy strands is to provide strands of fibers that are linear in disposition but have limited stretch in the longitudinal direction and are embedded in the polyurethane to control the stretch. However, in each embodiment of the invention disclosed in such patent, the stretch limiting means is provided across the entire cross section of the belt.
I have discovered, however, that if a stretch limiting means is provided across the entire width of a conveyor belt and the belt is tensioned to the predetermined limit, the belt cannot traverse horizontal curved because the outer edge of the belt in the curve cannot elongate to the extent necessary to traverse the curve.
The subject invention is directed toward an improved conveyor belt having an internal stretch limiting means disposed only in the center portion of the belt to allow for sufficient tensioning thereof to a predetermined extent while allowing the belt edges to elongate sufficiently to traverse horizontal curves.